Growth factors are polypeptides which stimulate a wide variety of biological responses (e.g. DNA synthesis, cell division, expression of specific genes, etc.) in a defined population of target cells. A variety of growth factors have been identified, including the transforming growth factor beta family (TGF-βs), epidermal growth factor and transforming growth factor alpha (the TGF-αs), the platelet-derived growth factors (PDGFs), the fibroblast growth factor family (FGFs) and the insulin-like growth factor family (IGFs), which includes IGF-I and IGF-II. Many growth factors have been implicated in the pathogenesis of cancer.
IGF-I and IGF-II (the “IGFs”)are related in amino acid sequence and structure, with each polypeptide having a molecular weight of approximately 7.5 kilodaltons (kDa). IGF-I mediates the major effects of growth hormone, and is thus the primary mediator of growth after birth. IGF-I has also been implicated in the actions of various other growth factors, since the treatment of cells with such growth factors leads to increased production of IGF-I. In contrast, IGF-II is believed to have a major role in fetal growth. Both IGF-I and IGF-II have insulin-like activities (hence their names), and are mitogenic (stimulate cell division).
IGF-I has been found to stimulate the growth of cells from a number of different types of cancer (Butler et al., 1998 Cancer Res. 58(14):3021-3027; Favoni R E, et al., 1998, Br. J. Cancer 77(12): 2138-2147). Additionally, IGF-I has additionally been found to exert anti-apoptotic effects on a number of different cell types, including tumor cells (Giuliano M, et al., 1998 Invest Ophthalmol. Vis. Sci. 39(8): 1300-1311; Zawada W M, et al., 1998, Brain Res. 786(1-2): 96-103; Kelley K W, et al., 1998, Ann. N.Y. Acad. Sci. 840: 518-524; Toms S A, et al., 1998, J. Neurosurg. 88(5): 884-889; Xu F, et al., 1997, Br. J. Haematol. 97(2): 429-440). U.S. Pat. No. 5,681,818 claims the administration of IGFBP-3 for the treatment of cancer.
A number of variant forms of IGF-I have been created which have altered binding characteristics for the IGF receptors, the insulin receptor, or IGFBP's (Cascieri et al. (1988) Biochemistry 27:3229-3233 and (1989) Jr. Biol. Chem. 264:2199-2202; Bayne et al. (1990) J. Biol. Chem. 265:15648-15652); Baxter et al. (1992) J. Biol. Chem. 267:60-65). Additionally, International Patent Application No. WO 97/39032 discloses the use of certain variant forms of IGF-I for the treatment of conditions where increased IGF-I activity is desired, such as diabetes, osteoporosis, and the like. The variant forms of IGF-I are proposed to displace IGF-I from IGFBP, resulting in increased IGF-I activity.
Almost all IGF circulates in a non-covalently associated complex of IGF-I, insulin-like growth factor binding protein 3 (IGFBP-3) and a larger protein subunit termed the acid labile subunit (ALS), such that very little free IGF-I is detectable. The ternary complex is composed of equimolar amounts of each of the three components. ALS has no direct IGF-binding activity and appears to bind only to the IGF/IGFBP-3 complex (Baxter et al., J. Biol. Chem. 264(20):11843-11848, 1989), although some reports suggest that IGFBP-3 can bind to rat ALS in the absence of IGF (Lee et al., Endocrinology 136:4982-4989, 1995). The ternary complex of IGF/IGFBP-3/ALS has a molecular weight of approximately 150 kDa and has a substantially increased half-life in circulation when compared to binary IGF/IGFBP-3 complex or IGF alone (Adams et al., Prog. Growth Factor Res. 6(24):347-356; presented October 1995, published 1996). This ternary complex is thought to act “as a reservoir and a buffer for IGF-I and IGF-II preventing rapid changes in the concentration of free IGF” (Blum et al (1991), “Plasma IGFBP-3 Levels as Clinical Indicators” in MODERN CONCEPTS OF INSULIN-LIKE GROWTH FACTORS, pp. 381-393, E. M. Spencer, ed., Elsevier, N.Y.). While there is essentially no excess (unbound) IGFBP-3 in circulation, a substantial excess of free ALS does exist (Baxter, J. Clin. Endocrinol. Metab. 67:265-272, 1988).
It should be noted that, while IGFBP-3 is the most abundant of the IGF binding proteins (“IGFBPs”), at least five other distinct IGFBPs have been identified in various tissues and body fluids. Although these proteins bind IGFs, they originate from separate genes and have distinct amino acid sequences. Unlike IGFBP-3, other circulating IGFBPs are not saturated with IGFs. IGFBP-3 and IGFBP-5 are the only known IGFBPs which can form the 150 kDa ternary complex with IGF and ALS. The IGF and ALS binding domains of IGFBP-3 are thought to be in the N-terminal portion of the protein, as N-terminal fragments of the protein isolated from serum retain these binding activities. However, some of the other IGFBPs have also been suggested for use in combination with IGF-I as therapeutics.
In addition to its role as the major carrier protein for IGF in serum, IGFBP-3 has been recently shown to have a number of different activities. IGFBP-3 can bind to an as-yet unidentified molecule on the cell surface, where it can inhibit the activity of exogenously-added IGF-I (Karas et al., 1997, J. Biol. Chem. 272(26):16514-16520). Although the binding of IGFBP-3 to cell surfaces can be inhibited by heparin, the unidentified cell surface binding molecule is unlikely to be a heparin-like cell surface glycosaminoglycan, because enzymatic removal of heparin glycosaminoglycans has no effect on IGFBP-3 cell surface binding (Yang et al., 1996, Endocrinology 137(10):4363-4371). It is not clear if the cell surface binding molecule is the same or different than the IGFBP-3 receptor that was identified by Leal et al. (1997, J. Biol. Chem. 272(33):20572-20576), which is identical to the type V transforming growth factor-beta (TGF-β) receptor.
IGFBP-3 has also been found to promote apoptosis. Interestingly, IGFBP-3 has been shown to promote apoptosis in cells with and without functional type 1 IGF receptors (Nickerson et al., 1997, Biochem. Biophys. Res. Comm. 237(3):690-693; Rajah et al., 1997, J. Biol. Chem. 272(18):12181-12188). However, there are conflicting reports as to whether apoptosis is induced by full length IGFBP-3 or a proteolytic fragment of IGFBP-3 (Rajah et al., ibid; Zadeh et al., 1997, Endocrinology 138(7):3069-3072).
IGF-I and IGFBP-3 may be purified from natural sources or produced by recombinant means. For instance, purification of IGF-I from human serum is well known in the art (Rinderknecht et al. (1976) Proc. Natl. Acad. Sci. USA 73:2365-2369). Production of IGF-I by recombinant processes is shown in EP 0 128 733, published in December of 1984. IGFBP-3 may be purified from natural sources using a process such as that shown in Baxter et al. (1986, Biochem. Biophys. Res. Comm. 139:1256-1261). Alternatively, IGFBP-3 may be synthesized by recombinantly as discussed in Sommer et al., pp. 715-728, MODERN CONCEPTS OF INSULIN-LIKE GROWTH FACTORS (E. M. Spencer, ed., Elsevier, N.Y., 1991). Recombinant IGFBP-3 binds IGF-I in a 1:1 molar ratio.
Topical administration of IGF-I/IGFBP-3 complex to rat and pig wounds is significantly more effective than administration of IGF-I alone (Id.). Subcutaneous administration of IGF-I/IGFBP-3 complex to hypophysectomized, ovariectomized, and normal rats, as well as intravenous administration to cynomolgus monkeys, “substantially prevents the hypoglycemic effects” of IGF-I administered alone (Id.).
The use of IGF/IGFBP-3 complex has been suggested for the treatment of a wide variety of disorders (see, for example, U.S. Pat. Nos. 5,187,151, 5,527,776, 5,407,913, 5,643,867, 5,681,818 and 5,723,441, as well as International Patent Applications Nos. WO 95/03817, WO 95/13823, and WO 96/02565. IGF-I/IGOFBP-3 complex is also under development by Celtrix Pharmaceuticals, Inc., as a treatment for several indications, including recovery from burns and recovery from hip fracture surgery.
While there are a large number of cytotoxic drugs available for the treatment cancer, these drugs are generally associated with a variety of serious side effects, including alopecia, leukopenia, mucositis. Accordingly, there is a need in the art for cancer therapies that do not induce the serious side effects associated with conventional cytotoxic chemotherapy.